The long-term objectives of this project are to produce novel vaccines and immunotherapeutics designed to abrogate the activities of poxvirus-encoded extra-cellular accessory proteins (XAPs). We will focus on the XAPs that interfere with host responses, particularly immune responses, against poxvirus infections. We will test the hypothesis that preexisting immunity to the XAPs will abrogate the in vivo replicative advantages that poxviruses gain from expressing these XAPs, and reduce viral virulence in the immunized host. The specific aims of this project are as follows: 1. Construct Venezuelan Equine Encephalitis (VEE) virus replicons expressing mutationally inactivated XAPs of cowpox virus (CPXV). Initially, we will focus on the extra-cellular viral inhibitors of complement and interferon. Subsequently, we will target the other orthopoxvirus XAPs, each of which is encoded by CPXV. 2. Assemble and test prototype VEE replicon particle (VRP) vaccines for CPXV-XAPs: VRPs will be assembled containing replicons expressing the genetically inactivated XAPs. We will determine the immunological responses of mice vaccinated with the VRP-CPXV-XAP vaccines. We will determine the abilities of antisera from immunized mice to neutralize the in vitro activities of the target XAPs, as well as cross-protective properties of the VRP-CPXV-XAP-induced antibodies against accessory proteins encoded by other orthopoxviruses, including variola virus. 3. Evaluate the efficacy of prophylactic VRP-CPXV-XAP vaccines in the mouse model: We will determine if vaccination with the VRP-CPXV-XAP vaccines can reduce CPXV replication and pathogenesis following challenge with CPXV in the mouse. We will also examine the effects of these VRP vaccines on infections with vaccinia viruses, and we will determine the efficacy of prime boost regimens involving primary immunization with VRP-CPXV-XAP vaccines followed by vaccination with vaccinia virus vaccines. 4. Evaluate the efficacy of therapeutic VRP-CPXV-XAP vaccines and anti-XAP antibodies in the mouse model: We will determine if post-infection vaccination with VRP-CPXV-XAP vaccines or post-infection treatment with monoclonal antibodies against XAPs encoded by cowpox virus can attenuate cowpox and vaccinia virus infections in the mouse.